1. Field
The embodiments described herein relate generally to systems for generating megavoltage radiation. More particularly, the described embodiments relate to the determination of megavoltage scatter radiation using one or more radiation-attenuating elements.
2. Description
A linear accelerator produces electrons or photons having particular energies. In one common application, a linear accelerator generates a radiation beam exhibiting megavoltage energies and directs the beam toward a target area of a patient. The beam is intended to destroy cells within the target area by causing ionizations within the cells or other radiation-induced cell damage.
Radiation treatment plans are intended to maximize radiation delivered to a target while minimizing radiation delivered to healthy tissue. A radiation treatment plan designer must assume that relevant portions of a patient will be in particular positions relative to a linear accelerator during delivery of the treatment radiation. The goals of maximizing target radiation and minimizing healthy tissue radiation may not be achieved if the relevant portions are not positioned in accordance with the treatment plan during delivery of the radiation. More specifically, errors in positioning the patient can cause the delivery of low radiation doses to tumors and high radiation doses to sensitive healthy tissue. The potential for misdelivery increases with increased positioning errors.
Imaging systems may be used to verify patient positioning prior to the delivery of treatment radiation. According to some examples, a radiation beam is emitted by a linear accelerator prior to treatment, passes through a volume of the patient and is received by an imaging system. The imaging system produces a set of data that represents the attenuative properties of objects of the patient volume that lie between the radiation source and the imaging system.
The set of data is used to generate a two-dimensional portal image of the patient volume. The portal image will include areas of different intensities that reflect different compositions of the objects. For example, areas of low radiation intensity may represent bone and areas of high radiation intensity may represent tissue. Several two-dimensional portal images may be acquired from different perspectives with respect to the patient volume and combined to generate a three-dimensional image of the patient volume. The foregoing images may be used to diagnose illness, to plan radiation therapy, to confirm patient positioning prior to therapy, and/or to confirm a shape and intensity distribution of a radiation field prior to therapy.
The imaging system receives scatter radiation during acquisition of the above-described portal images. Such scatter radiation does not travel along an expected radiation trajectory from the radiation source to the imaging system. In other words, scatter radiation received at a particular location of the imaging system does not reflect attenuative properties of all the tissues located along an expected trajectory from the radiation source to the particular location. As a result, received scatter radiation induces noise and reduces the intensity gradients (i.e. contrast) between image areas that represent different objects in a portal image. The reduced contrast may inhibit identification of structures within the portal image, particularly with respect to soft tissue structures.
Conventional single-row (one-dimensional) imaging systems include a row of radiation detectors to detect kilovoltage radiation. These systems may include thin metal collimators to prevent scatter radiation from reaching the radiation detectors. Such techniques are impractical for two-dimensional imaging systems employing thousands of detectors. The techniques are particularly impractical for megavoltage radiation-based imaging due to the collimator masses that would be required to block megavoltage scatter radiation.
Some conventional kilovoltage radiation-based imaging systems determine scatter radiation by acquiring a first image of an object with an imaging device while an array of dense cylindrical elements lies between a kilovoltage radiation source and the object. The elements prevent primary photons from the radiation source from reaching the imaging device and therefore produce shadows within the first image. The shadows are assumed to be uniformly absent of non-scatter radiation and any photon fluence detected in the shadows is therefore assumed to have undergone scatter. Scatter radiation may therefore be measured based on fluence within the shadows and the foregoing assumptions.
The above-described approach is not suitable for high-energy systems. For example, the cylindrical elements used in conventional kilovoltage radiation-based imaging systems would not sufficiently or uniformly attenuate photons of a megavoltage radiation beam. Accordingly, the aforementioned assumptions would not be valid and any measurements of scatter radiation based on such assumptions would be unsuitably inaccurate.
It would therefore be beneficial to provide an efficient system to determine an amount of scatter radiation resulting from irradiation of an object with megavoltage radiation. Such a determination may facilitate efficient reduction of scatter-induced noise within an image of the object.